Add initial execution progress streaming impl

[bduffany]

• Adds a Publisher interface that lets us publish the new progress updates concurrently with other progress updates. We need this because there's a loop in ExecuteTaskAndStreamResults which periodically publishes progress updates while the task is executing, but we also want to publish fine-grained progress events while this loop is ongoing.
• Publish the basic progress updates: pulling image, downloading inputs, executing, uploading outputs.
• Update bb execute to log debug messages with these progress updates.
• Some small refactoring in the rexec / operation packages.

TODO in future PRs:

• Publish progress for VM boot / snapshot resume.
• Don't publish "pulling image" status if we're not actually pulling an image.

Screenshot showing it in action (with bb execute for now as a simple client):

rec-2024-05-09_18.28.59.mp4

Related issues: N/A

[maggie-lou]

uber-nit: Could you rename er? It looks like a typo of err

[bduffany]

haha it does 😆 done